Based on the installation for the large cantilever steel structure of Podiums in Wuhan Optics Valley Science and Technology Mansion, The paper introduced the application of the unsupported overhead in-situ installation technology in the installation of cantilever steel structure. The installation of cantilever steel structure in this project need to be implemented after the completion of installation of non cantilever structure in podiums. At that time, the narrow construction site and difficultly of lifting points setting cannot meet the assembling on the ground and integral lifting. In addition, the high-position of installation of cantilever structure caused high-cost to set temporary support frame and the stability of support frame cannot be ensured. In concrete construction, using without support overhead in-situ installation technology, analyzing and utilizing stress characteristics of cantilever steel structure, assisting location and stability of super heavy components by means of temporary pull rod and imitating the internal force and deformation in hoisting process by means of finite element software, according to the principle materials first and then auxiliary materials and inside-out lifting sequence making the scientific lifting sequence and forming the own stress system of cantilever structure step by step. The installation of cantilever steel structure in this project ensure construction security and economize the cost. Also, quality control and monitor, arrangement of construction procedures and security measures were discussed in the paper. The accuracy of cantilever steel structure after completion meet the design and standard requirements, the scientific nature and security of programme were verified in concrete construction.